Remembering the audacity of the twin towers

The soaring twin towers of the World Trade Center became an affirmation of the American value of dreaming big. To the engineer who designed them, their loss on 9/11 remains heartbreaking, but he's found the resilience to keep dreaming.

Skyline view of 1996 Manhattan shows the World Trade Center's twin towers rising above lower Manhattan. The North Tower was completed in 1972, the South Tower in 1973.

Liza M. Greene/AP

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They were too large, too sterile, and too indifferent to their surroundings. But the World Trade Center towers were also a reminder of what America could achieve. Conceived at the same time President Kennedy committed to putting Americans on the moon, they would become an affirmation of the value of thinking large.

Their demise on 9/11 has been deeply mourned, though not as grievously as the innocent lives lost that day. With their sleek lines and audacity to rise a quarter-mile into the sky, the towers did eventually win the adoration of most of New York and her visitors. Who among us has watched a pre-9/11 movie and not felt a stab upon seeing the New York skyline with the two iconic sentinels intact?

Like many things dreamed of in the 1960s – the Great Society, "Model Cities," a new civil rights law – the towers were beyond anything builders had ever contemplated. Each of the 110 stories in each structure was nearly an acre. The buildings married 200,000 tons of steel, 425,000 cubic yards of concrete, more than 43,000 windows, 198 miles of heating and cooling ducts, 12,000 miles of electric cables, and more than 100 elevators. Ten thousand people worked on the towers. The World Trade Center would take 10 years to realize, and when it opened in the early 1970s, it was so large it had its own ZIP Code.

In 1962, Minoru Yamasaki was selected as the architect. Few outside the project, however, knew at the time that the steel structure that made his vision possible was conceived by a relatively unknown Seattle-based engineering firm, Worthington, Skilling, Helle and Jackson. The partner-in-charge was John Skilling, and the lead structural engineer was 34-year-old Leslie Robertson.

To Mr. Robertson, “the Trade Center was a matter of expanding the basic ideas of structure." It was to be much more.

Based on accepted engineering practice, skyscrapers had topped out in 1931 with the Empire State Building's 102 floors of sequentially diminishing floor space. Robertson was going to rewrite that limitation, with each tower providing more than twice the area contained in the Empire State Building.

The young engineer had already demonstrated an aptitude for solving challenging problems. He designed uniquely fortified foundations for a 5-mile-long bridge. He was among the first to use cross-bracing to build more efficiently. And he designed liquid-filled structural columns to fireproof exposed steel.

The largest concern in engineering the World Trade Center was the wind, an even greater force than the downward load of the buildings, explained Robertson in a recent interview. Unlike traditional skyscrapers that relied on a dense grid of columns and beams, Robertson's and Skilling's solution thoroughly reconfigured the towers’ support. The two men designed a dense row of columns around the perimeter of each structure and another set of columns circumscribing the buildings' core. Long prefabricated floor trusses connected the two sets of columns.

In creating far more open floor space than in traditional skyscrapers, this structural configuration required the exterior walls to carry extraordinary weight. According to Robertson, this made them "even more robust than traditional skeletal walls to counter the lateral force of the wind."

To control the sense of movement inside (for the buildings needed to sway slightly to remain standing), his team, along with the 3M Corporation, devised and installed 11,000 unique viscoelastic dampers, i.e., structural shock absorbers.

The World Trade Center marked the first use of computer modeling to forecast how a structure would perform, leading to more precise design specifications of materials and construction. Robertson also used this technology to confirm the structures could withstand a hit by the largest plane of the time – a Boeing 707.

No one involved with the World Trade Center, however, was able to project what would happen to the jet fuel.

The National Institute of Standards and Technology’s investigation of the collapse of the World Trade Center – the official government study – found "computing resources and software necessary to conduct these analyses did not exist in the 1960s." Still, NIST concluded that if the structures’ fireproofing hadn’t been blown off by the impact of the jets, the towers would likely have remained standing, just as Robertson said.

Since 9/11, some studies, including one issued by the Federal Emergency Management Agency and the American Society of Structural Engineers, celebrated the buildings for having stood after impact for as long as they did, giving so many occupants time to exit. Engineers cited in the documentary series "NOVA" said, in the program "Why the Towers Fell," the search for efficiency may have produced structures that could have been more susceptible to progressive failure. Ten years after the disaster, the subject remains contentious and difficult to discuss dispassionately.

Prof. Robert Bea of the University of California, Berkeley – one of the country's leading forensic engineers who is heading the investigation of the Macondo oil well blowout in the Gulf of Mexico last year – describes Robertson's design as excellent. "One part of me as an engineer looks at that efficiency achieved and says, 'Well that's exactly what we should be doing.'

"However," he adds, "it has taken me my entire professional life to learn that anyone's structural system cannot, will not be perfect. Things will not perform as you expect them to. Things will not be built as you expect them to. The system has to have a level of protection called robustness that allows it to tolerate damage and defect."

According to Robert Prieto, chairman of the engineering firm Parsons Brinckerhoff, "Unless we are prepared to live in an engineered environment that resembles the complex of caves in Afghanistan, we will not design buildings to stop planes."

The loss of the World Trade Center ultimately came down to this: The hatred driving a handful of individuals exceeded the imagination, intelligence, and commitment that had led thousands of men and women to create two remarkable towers.

This is not much solace for the families of the thousands who perished on 9/11, or for Robertson. Many did come to him in the months that followed, looking for some kind of answer, which he knew he could not provide.

He remembers "the first was a young woman, perhaps 13 or 14 years old. Her brother was working on one of the high floors. We met in a park at the foot of Manhattan. The tears came as her body shook. And as we cried together, words were not required."

It may have been through embraces and tears shed with strangers that Robertson, too, partially came to terms with the incomprehensible. A career he thought was over continues to thrive, and Robertson, now an octogenarian, has gone on to design some of the world's tallest skyscrapers. Among them: the 101-story Shanghai World Financial Center, completed in 2008, and the Lotte Jamsil Tower in Seoul, currently under construction, which will be 123 stories.

In the 10 years since 9/11, America has been sorely tested by two wars, a severe recession, and staggering debt. And what of its ability to dream as it once did? Time will tell, but perhaps Robertson's own story of resilience offers a clue.